Device and method for emitting a light beam intended to form an image, projection system, and display using said device

ABSTRACT

The invention concerns a device for emitting a light beam intended to form an image, the device comprising one or more sources, each emitting a laser-type beam, the device being designed to form the light beam from the one laser beam or from a combination of the laser beams, the device comprising attenuation means, disposed downstream of the source(s), enabling the optical power of the light beam to be varied, the attenuation means comprising one or more retractable filters which can be interposed in the beam or beams.

The invention relates to a device and a method for emitting a light beamintended to form an image, a projection system using said device and adisplay using said system.

The invention will have applications, for example, in motor vehicles forinforming the users of the vehicle, in particular its driver.

It is known to equip a motor vehicle with a so-called head up displaysystem. Such a system is placed in the field of vision of the motorvehicle driver and displays information relating to the status of themotor vehicle, the traffic or other.

In order not to interfere with driving, it is necessary that thebrightness of the projected image is adapted to the ambient brightness.In particular, it is necessary that the optical power of the projectedimage is greatly different during a daytime journey and a journey atnight or when passing through a tunnel.

With head up displays using light emitting diodes, it has already beenproposed to vary the brightness of the projected image by controllingthe supply current of the diodes. Such devices allow a satisfactoryadaptation of the brightness. However, their daytime brightness remainsinadequate.

In order to compensate for this defect, displays are known which uselight sources of the laser type. However, the adaptation of thebrightness provided in such a case by controlling the amount of currentsupplying the sources does not make it possible to sufficiently reducethe optical power of the image. By way of example, if it is consideredthat it is desired to have an optical power of the order of 10000 Cd/m2in daytime driving condition and of the order of 5 Cd/m2 in nightdriving conditions, it is observed that it is necessary to reduce thebrightness by a factor of 2000 when changing from one of the drivingconditions to the other.

Research conducted by the applicant on adapting the brightness of theprojected image by controlling the current supply of the light sources,for example using pulse width modulation technologies or control of theamplitude of the current used, shows that these technologies havelimits. It is notably necessary to have a minimum pulse duration and aminimum amplitude of the current for the linear operation of the diodes.

The purpose of the invention is to solve the above problems by proposinga device for emitting a light beam intended to from an image, saiddevice comprising one or more sources, each emitting a beam of the lasertype, said device being configured to form said light beam from the onelaser beam or by bringing said laser beams together by combination.“Bringing together” means that, to the naked eye, said laser beams form,after combination, a single light beam even though they are stillpresent individually in said beam. For the purpose of simplicity, theterm “light beam” will continue being used hereafter to denote thecombination of the laser beams after bringing them together.

According to the invention, said device comprises attenuation means,situated downstream of said source or sources, making it possible tovary the optical power of the light beam, said attenuation meanscomprising one or more retractable filters, able to be interposed insaid beam or beams. The term “optical power” means the light power ofthe light flux emitted by the source or sources. It can equally well bemeasured for example in milliwatts (mW), from the current generated by aphotodiode receiving the light flux, or in candela per square meter(Cd/m2), or from the illumination produced by the light flux on a givenarea.

By attenuating the light beam emitted by the laser sources by specificmeans operating in a way other than by controlling their current supply,greater possibilities are available for reducing the optical power.Consequently, it is possible to simultaneously benefit from high daytimebrightness and low brightness at night.

According to one aspect of the invention, said device comprises anactuator, able to displace said or each of said filters with respect tosaid beam or beams.

Said filter will be able to exhibit a progressive filtering capacity.According to a first embodiment, said filter is a plate exhibiting afiltering capacity which increases in a given direction D and saidfilter is mobile in said given direction. According to anotherembodiment, said filter is a disk exhibiting a filtering capacity whichincreases angularly about the center of said disk and said filter ismobile in rotation about the center of said disk.

According to another aspect of the invention, which can be combined withthe preceding one, said emission device can comprise means of control ofthe current supply of said sources, said control means being configuredto provide additional adjustment of the optical power of said lightbeam. In this way the capabilities of adjustment of the optical powerprovided by the attenuation means and by the control means are combinedin order to achieve the desired attenuation factor, in particular atleast 1000, or even 2000.

Said control means can be configured to provide linear currentregulation of the optical power of said laser beams in order to providea choice of color of the light beam according to a proportion of opticalpower allocated to each of said laser beams. In this way a method ofregulation dedicated to choice of color is available.

Said control means can also be configured to provide regulation by pulsewidth modulation of the optical power of said laser beams in order toachieve said additional adjustment of the optical power of said lightbeam. In this way a method of regulation dedicated to the additionaladjustment of the optical power is available. In other words, theattenuation factor can result from the combined action of saidattenuation means and a pulse width modulation of the supply current ofsaid laser sources, said modulation being carried out using said controlmeans.

More precisely, said control means are configured so that said pulsewidth modulation regulation provides said additional adjustment, forexample, according to an attenuation factor of between 5 and 20, notablyof 10. As for the attenuation means, these can make it possible toobtain an attenuation factor of between 100 and 300, notably of about200, in particular by disposing several said filters in series.

According to other features of the invention which can be taken togetheror separately, possibly combined with all at parts of the precedingfeatures:

-   -   said attenuation means are configured to vary the optical power        of the at least one of said laser beams before its combination        with at least one of said other laser beams, and/or said        attenuation means are configured to vary the optical power of        said light beam coming from the combination of said laser beams,    -   said device furthermore comprises means of detecting ambient        brightness and processing means making it possible to act on        said attenuation means as a function of the ambient brightness        detected by said detection means.

Advantageously, said emission device comprises at least three lasersources, each one of the three sources being capable of emitting amonochromatic light beam.

This being so, the invention also relates to an image projection systemcomprising a device for the emission of a light beam such as describedabove.

Said system comprises, for example, means for forming an image from saidlight beam. Said attenuation means can be situated upstream and/ordownstream of said image forming means.

Said image forming means comprise, for example, a diffuser screen uponwhich is formed an image produced from the light source or sources ofsaid emission device.

According to a first implementation, said image forming meansfurthermore comprise a laser scanning projector generating a light beamforming an image on the diffuser screen from the light beam generated bysaid emission device.

According to another embodiment, said image forming means comprise aprojector comprising an array of micromirrors interposed between thelight source or sources and the diffuser screen, the array ofmicromirrors forming a first projected image on the diffuser screenwhere a second projected mage is formed.

The invention also covers a head up display system comprising aprojection system such as described above.

Such a system comprises, for example, a semi-reflecting plate, able tobe disposed between the steering wheel and the windscreen of a vehicle,or the windscreen as such.

The system according to the invention can furthermore comprise areflecting device interposed in the path of the image between thediffuser screen and the semi-reflecting plate. Such a reflecting devicemakes it possible to install the projection device more easily whilstsending the image to the desired place from a given location of theemission device.

The invention also relates to a method for emitting a light beamintended for forming an image, in which method there is provided one ormore sources, each one emitting a beam of the laser type, said lightbeam is formed from the one laser beam or by bringing together saidlaser beams by combination and the optical power of said light beam isvaried using attenuation means situated downstream of said source orsources.

It will also be possible to provide additional adjustment of the opticalpower of said light beam using means for controlling the current supplyof said source or sources, notably according the features alreadydescribed above.

Advantageously, said method uses the emission device described above.

Other features, details and advantages of the invention will emerge moreclearly on reading the description given hereafter as an indication andwith reference to the drawings in which:

FIG. 1 is a diagrammatic view of an emission device according to theinvention,

FIG. 2 is a diagrammatic view of a first type of filter of the deviceshown in FIG. 1,

FIG. 3 is a diagrammatic view of a second type of filter of the deviceshown in FIG. 1,

FIG. 4 is a diagrammatic view of a head up display system according tothe invention.

It should be noted that the figures disclose the invention in a detailedmanner in order to use the invention, it being possible of course forsaid figures to be used for better defining the invention as necessary.

As shown in FIG. 1, the invention firstly relates to a device 1 foremitting a light beam, intended to form an image. Such a device is moreparticularly intended to equip a head up display of a motor vehicle,where at least one item of information related to the vehicle isprojected.

Said device comprises one or more sources 4, 5, 6 each emitting a beam7, 8, 9 of the laser type. They are, for example, laser sources,typically laser diodes, each laser source emitting a monochromatic beam,that is to say consisting of a single color.

Said device here comprises a plurality of sources 4, 5, 6, three of themin this case, said device being configured to form the light beam 10 bybringing the beams 7, 8, 9 individually emitted by each of said sources4, 5, 6 together in combination. More precisely, it will be possible forthe sources to be ones emitting a different color from one source toanother. The colors are, for example, red, green or blue (RGB).

The optical power of each of the sources is controlled independently, byusing the supply current of the laser source or sources. At a givenoptical power, the color of the light beam 10 is determined by the wayin which a power ratio is established between the different laserdiodes. For example, in order to obtain white light, the optical powers,in proportion, must be established according to the followingdistribution: 60 for the green diode, 30 for the blue diode, 10 for thered diode. As described below, the optical power of each of the sourcescan also be controlled in order to modulate the optical power of thelight beam 10.

The beams 7, 8, 9 emitted by each of the sources are oriented, forexample, parallel with each other and reflect in the same direction inorder to form said common light beam 10 by combination. To this end,said device 1 here comprises optical elements that are semi-transparent,over a range of wavelengths, such as dichroic mirrors or combiningplates 11, intercepting the beams 7, 8, 9 emitted by each of saidsources and combining them in the direction of said beam 10.

More generally, said device 1 is configured to form said light beam 10from said laser beam or beams 7, 8, 9, however many sources 4, 5, 6 maybe involved. In the case of a single source, the light beam 10 iscomposed of the laser beam emitted by the single source used and theimage obtained will then be monochromatic, composed of the differentoptical powers applied to each of the points forming it, according to agradation of said color. In the case of a plurality of sources,typically the three sources 4, 5, 6 mentioned above, said common beam 10which then forms said light beam will allow the establishment of animage according to a color spectrum whose resolution will correspond tothe fineness of control of the power supply of said sources 4, 5, 6.

According to the invention, said device 1 comprises attenuation means12, situated downstream of said source or sources 4, 5, 6, making itpossible to vary the optical power of the light beam 10. Saidattenuation means 12 accordingly comprise one or more retractablefilters 20, able to be interposed in said beam or beams 7, 8, 9, 10.Here, provision is made for a single filter 20 in the trajectory of thelight beam 10. In other words, a color and/or an intensity beingconferred on the light beam 10 by controlling the current supply of thesources, said attenuation means 12 make it possible to vary the opticalpower of the beam or beams 7, 8, 9, 10 as a function of the position ofsaid filter or filters with respect to the latter. In particular, itwill be possible to adapt the optical power of the beam to daytimedriving conditions and to night driving conditions.

Said device can comprise means of controlling the current supply of saidsources. As mentioned above, they make it possible to choose the colorof the light beam 10.

More precisely, said control means are configured, for example, toprovide linear current regulation of the optical power of said laserbeams 7, 8, 9 in order to provide said choice of color of the light beam10, according to a proportion of optical power allocated to each of saidlaser beams 7, 8, 9. It is possible, for example, to provide an encodingof the color in six bits, corresponding to sixty four levels of opticalpower for each of said laser beams 7, 8, 9.

Said control means can also be configured to provide an additionaladjustment of the optical power of said light beam. A particularly highattenuation rate can be achieved in this way.

More precisely, said control means are configured to provide regulationby pulse width modulation of the optical power of said laser beams 7, 8,9 in such a way as to achieve said additional adjustment of the opticalpower of said light beam 10, notably according to an attenuation factorof between 5 and 20, in particular of about 10.

In this way it is possible to adjust the color and/or the optical powerof said light beam 10. Said control means comprise, for example, amicrocontroller, which is not shown.

As illustrated in FIGS. 2 and 3, said filter 20 exhibits a progressivefiltering capacity.

More precisely, according to the embodiment of FIG. 2, said filter 20 isa plate exhibiting a filtering capacity which increases in a givendirection D. Said plate is, for example, rectangular, and exhibits anincreasing opacity making it possible to have a filtering capacity whichgrows along the length of the rectangle. Said filter 20 is furthermoremobile in translation in said given direction. Said plate is oriented ina manner transverse, in particular orthogonal, to the beam and it isunderstood that, according to the position of said plate in the givendirection, the filtering capacity of the filter 20 and therefore theattenuation factor of the beam vary, passing from an attenuation ratecorresponding to almost complete transmission of the beam at one end 21a of the plate to an attenuation rate corresponding to very weaktransmission of the beam at the other end 21 b.

According to the embodiment of FIG. 3, said filter 20 is a diskexhibiting a filtering capacity which increases angularly about thecenter of said disk. Said filter 20 is furthermore mobile in rotationabout the center of said disk. Said disk is oriented in a mannertransverse, in particular orthogonal, to the beam and it is understoodthat, according to the angular position of said disk, the filteringcapacity of the filter 20 and therefore the attenuation factor of thebeam vary, passing from an attenuation rate corresponding to almostcomplete transmission of the beam to an attenuation rate correspondingto very weak transmission of the beam for a given rotation of said disk,here by 360°.

Referring again to FIG. 1, it is noted that said device will furthermorebe able to comprise an actuator 22, able to displace said filter or eachof said filters 20 with respect to said beam. According to the previousembodiments, this may entail, respectively, a micro ram or a steppermotor.

Said attenuation means, by virtue of said filter or by placing severalfilters in series, will be able to make it possible to obtain anattenuation factor of about 200 which, combined with the attenuationfactor coming from the control means, in particular from the pulse widthmodulation, makes it possible to reach the level of 2000 mentionedabove.

This being so, according to the configuration shown, said attenuationmeans 12 are placed in the path of the common light beam 10. In otherwords, said attenuation means 12 are configured to vary the opticalpower of said common light beam 10. More precisely, as alreadymentioned, said filter or filters are placed in the path of the commonlight beam 10.

According to a variant which is not shown, said attenuation means 12 areplaced at least partially in the path of the beams emitted by thesources 4, 5, 6, in this case between said sources 4, 5, 6 and saidmirrors 11. In other words, said attenuation means 12 are configured tovary the optical power of the laser beam emitted by each of said sources4, 5, 6. More precisely, said filter or filters are placed between saidsources 4, 5, 6 and said mirrors 11. Of course, these two variants canbe combined.

Said device can furthermore comprise means of detection of ambientbrightness and processing means making it possible to act on saidattenuation means as a function of the ambient brightness measured bysaid detection means. Said means of detection of the ambient brightnesscomprise, for example, a light detector, such as the one modifying thedisplay screens on the dashboard of a vehicle. Said processing means areintegrated, for example, at the level of the microcontroller mentionedabove.

As shown in FIG. 4 the invention also relates to an image projectionsystem 100 comprising an emission device 1 such as described above. Theprojection system projection 100 furthermore comprises means 102 offorming an image from said light beam 10.

Said image forming means 102 comprise, for example, a scanning generator110 whose function is to move the light beam 10 horizontally andvertically for the purpose of producing scanning at a frequency, notablyequal to 60 Hz, as a non-limiting example. The scanning generator 110notably comprises a scanning mirror having a micro-electro-mechanicalsystem (hereafter called MEMS) upon which the light beam 10 is reflectedas a scanning beam 103. Such a MEMS mirror has for example a diameter of1 mm. The MEMS mirror is able to rotate about two axes of rotation inorder to produce a scanning, for example at a the refresh frequency of60 Hz, of a diffuser screen 111 of said image forming means 102. Saidimage is formed on said diffuser 111. Alternatively, the MEMS mirror canbe replaced by two movable flat mirrors, the movements of which arecoupled. One of these mirrors can be dedicated to scanning along ahorizontal axis whilst the other mirror can be dedicated to scanningalong a vertical axis.

The diffuser 111 where the image is formed can be a transparentprojection screen having a complex structure for projection bytransparency. Alternatively, it can be translucent. It is made, forexample, from glass, notably frosted, or from polycarbonate. By way ofexample, the diffuser screen is of the exit pupil (Exit Pupil Expander)type. It makes it possible to have a widened observation cone. Itextends in a plane traversed by the light beam, the image resulting fromthis scanning beam 103 being formed in the plane of a face of thediffuser screen 111.

This diffuser screen receives the scanning beam 103. It is arranged tocause dispersion of this scanning beam 103 in an angular sector, forexample, equal to 30° with respect to the direction of the scanning beam103 at the moment it strikes the diffuser screen 111. In order to dothis, according to a non-limiting example, a face 112 of the diffuserscreen is rough, in the sense that it comprises asperities which causethe dispersion of the scanning beam 103. The rough face 112 correspondsto the one though which the beam exits, that is to say the face uponwhich the image is formed.

According to another variant which is not shown, said image formingmeans do not comprise a scanning generator, such as previouslydescribed, but a micromirror array (also called digital micromirrorsystem). In this configuration the image is formed at the level of themicromirror array and then projected onto the diffuser screen. Ingeneral, projection optics are placed between the array and the screen.Each micromirror corresponds to a pixel of the image. In thisembodiment, the image is not formed for the first time on the diffuserscreen but an image previously formed on the micromirror array isreceived.

It should be noted that said attenuation means 12 can be, as in theexample shown, upstream of said image forming means 102. They can alsobe downstream. In a variant, they can be placed between the scanninggenerator 110 or the micromirror array, on the one hand, and thediffuser screen 111 on the other hand.

Said projection system can furthermore comprise various mirrors 104,106, notably in the path of the scanning beam 103.

The invention also relates to a display, notably a head up display,comprising a projection system 100 according to any one of the variantsdescribed above.

Downstream of the diffuser screen 111 in the direction of movement ofthe light beam, said display comprises at least a semi-reflective plate126 and a reflecting device 125 interposed in the path of the imagebetween the diffuser screen 111 and the semi-reflective plate 126. Inthis figure, the path of the image is symbolized by three arrows 30shown in dotted line which reflect on the reflection device 125 beforebeing displayed through the semi-reflective plate 126. The latter allowsan enlargement and/or, by transparency, a display of the image beyondsaid semi-reflective plate, notably beyond the windscreen of theequipped vehicle, at the level of a virtual screen 130, obtained withthe help of said semi-reflective plate 126.

This transparent plate has a reflection power at least equal to 20%,which allows the user to see the route taken by the vehicle through theplate, whilst benefitting from a high contrast making it possible to seethe displayed image. Alternatively, the display of the image can takeplace on the windscreen of the vehicle equipped with said display.

As already stated, said attenuation means can be situated downstream ofthe image forming device 102 and as far as the semi-reflective plate126.

1. A device for emitting a light beam intended to form an image, saiddevice comprising one or more sources, each emitting a beam of the lasertype, said device being configured to form said light beam from the onelaser beam or from the bringing together of said laser beams bycombination, said device comprising attenuation means, situateddownstream of said source or sources, making it possible to vary theoptical power of the light beam, said attenuation means comprising oneor more retractable filters, able to be interposed in said beam orbeams.
 2. The device as claimed in claim 1 comprising an actuator, ableto displace said filter or each of said filters with respect to saidbeam or beams.
 3. The device as claimed in claim 1 in which said filterexhibits a progressive filtering capacity.
 4. The device as claimed inclaim 1 in which said filter is a plate exhibiting a filtering capacitywhich increases in a given direction D and said filter is mobile in saidgiven direction.
 5. The device as claimed in claim 1 in which saidfilter is a disk exhibiting a filtering capacity which increasesangularly about the center of said disk and said filter is mobile inrotation about the center of said disk.
 6. The device as claimed inclaim 1 comprising means of control of the current supply of saidsources, said control means being configured to provide additionaladjustment of the optical power of said light beam.
 7. The device asclaimed in claim 6, in which said control means are configured toprovide linear current regulation of the optical power of said laserbeams in order to provide a choice of color of the light beam accordingto a proportion of optical power allocated to each of said laser beams.8. The device as claimed in claim 6, in which said control means areconfigured to provide regulation by pulse width modulation of theoptical power of said laser beams in order to achieve said additionaladjustment of the optical power of said light beam.
 9. The device asclaimed in claim 8, in which said control means are configured so thatsaid regulation by pulse width modulation provides said additionaladjustment according to an attenuation factor of between 5 and
 20. 10.The device as claimed in claim 1, in which said attenuation means areconfigured to vary the optical power of at least one of said laser beamsbefore its combination with at least one of said other laser beams. 11.The device as claimed in claim 1, in which said attenuation means areconfigured to vary the optical power of said light beam coming from thecombination of said laser beams.
 12. The device as claimed in claim 1,furthermore comprising means of detecting ambient brightness andprocessing means making it possible to act on said attenuation means asa function of the ambient brightness measured by said detection means.13. An image projection system comprising a device as claimed claim 1.14. The system as claimed in claim 13 comprising means for forming animage from said light beam and in which said attenuation means aresituated downstream of said image forming means.
 15. A display, notablya head up display, comprising at least a projection system as claimed inclaim
 13. 16. A method for emitting a light beam intended for foci lingan image, in which method there is provided one or more sources, eachone emitting a beam of the laser type, said light beam is formed fromthe one laser beam or by bringing together said laser beams bycombination and the optical power of said light beam is varied usingattenuation means situated downstream of said source or sources.
 17. Themethod as claimed in claim 16, in which additional adjustment of theoptical power of said light beam is provided using means for controllingthe current supply of said source or sources.
 18. The device as claimedin claim 2 in which said filter exhibits a progressive filteringcapacity.
 19. The device as claimed in claim 2 in which said filter is aplate exhibiting a filtering capacity which increases in a givendirection D and said filter is mobile in said given direction.
 20. Thedevice as claimed in claim 2 in which said filter is a disk exhibiting afiltering capacity which increases angularly about the center of saiddisk and said filter is mobile in rotation about the center of saiddisk.